1. Field of the Invention
This invention relates to a primer composition for improving the adhesion between a solid substrate and a thermoplastic. More particularly this invention relates to a primer composition consisting essentially of an epoxy or mercapto functional organosilicon compound and an alkoxymethyltriazine compound.
2. Description of the Prior Art
Organic plastics are divided into two categories: thermoplastic resins and thermosetting resins. The former are unreactive materials which soften on heating and harden on cooling without any appreciable formation of new chemical bonds. Thermosetting resins are reactive materials which harden on heating due to the formation of new chemical bonds which crosslink the material.
Organofunctional silanes which react with the thermosetting resins through the organofunctional group have been successfully used as primers to enhance the bond between many substrates and thermosetting resins. Thermoplastic materials generally do not react with the organofunctional group of a silane to form a chemical bond so that attempts to bond thermoplastics with silane primers are often disappointing. The adhesion of thermoplastics to substrates is further complicated by the tendency of water from the surroundings to rapidly deteriorate the bond strength.
One method of bonding thermoplastics is disclosed in U.S. Pat. No. 3,306,800 where a primer is employed which is an organic polymer having a molecular weight of at least 2,000 which is compatible with the thermoplastic and which has a silane grafted to each polymer molecule. Similarly, a glass coating material is described in U.S. Pat. No. 3,664,982 for improving the adhesion between glass fibers and thermoplastics. The coating material is a reaction product of a siloxane and a polyimine and may contain additionally a film forming material which is reactive with the polyimine such as hexamethoxymethylmelamine. The improved bonding is taught to result from the mechanical interlocking of the thermoplastic with the polyimine molecules, or molecules reacted therewith. It is shown that the combination of the film forming material and aminopropyltrimethoxysilane does not produce a comparable effect. The effect of water on these mechanical bonds is not discussed.
The present invention provides water resistant bonding of thermoplastics to solid substrates by use of a stable primer composition consisting essentially of an alkoxymethyltriazine and an epoxy or mercapto organofunctional silane. Although silanes such as 3-glycidoxypropyltrimethoxysilane have been used alone as primers or in combination with thermosetting resins as binders and impregnants, the concept of the above-described primer is believed to be novel. For example, U.S. Pat. No. 3,488,310 teaches that combining 3-glycidoxypropyltrimethoxysilane with a thermosetting urea-melamine-formaldehyde-glycol resin gives an improved impregnating composition for bonded glass fiber boards. The cured products have improved dry and wet tensile strength and a durable water repellent surface. Such thermosetting resin binders and impregnants do not suggest that the non-cured compositions of the instant invention would function as primers for improving adhesion between solid substrates and thermoplastics.